Self assembled molecules on immersion silver coatings

ABSTRACT

A method for enhancing the corrosion resistance of an article comprising a silver coating deposited on a solderable copper substrate is provided. The method comprises exposing the copper substrate having the immersion-plated silver coating thereon to an anti-corrosion composition comprising: a) a multi-functional molecule comprising at least one organic functional group that interacts with and protects copper surfaces and at least one organic functional group that interacts with and protects silver surfaces; b) an alcohol; and c) a surfactant.

REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. provisional application60/986,481 filed Nov. 8, 2007.

FIELD OF THE INVENTION

The present invention generally relates to methods and compositions fordepositing protective organic films on immersion silver coatings andelectrolytic plated silver, particularly immersion silver coatings oncopper substrates.

BACKGROUND OF THE INVENTION

For many years, bare boards comprising copper circuitry were finishedwith eutectic tin-lead solder coating according to the Hot Air SolderLeveling (HASL) process. Due to the Restriction of Hazardous Substances(ROHS) directive, the industry has moved away from using lead as acomponent of the final finish of bare boards.

Alternative final finishes include organic solderability preservative(OSP), electroless nickel-immersion gold (ENIG), immersion tin, andimmersion silver. OSP is an organic coating that is susceptible tochemical and mechanical removal and thus may not adequately protectcopper circuitry from oxidation. ENIG is vulnerable to common pollutantsand is sensitive to high humidity and tends to fail due to corrosion.Moreover, the process is slow and difficult to control. Finally, the useof gold renders it a relatively expensive process. Immersion tin issusceptible to the formation of copper-tin intermetallic and tin oxide.

A particular problem observed with immersion silver protective coatingsis creep corrosion of copper salts at certain bare copper interfacesbetween copper and silver. Immersion silver may not adequately covercopper surfaces for a variety of reasons. For example, immersion silverprocesses may not sufficiently coat copper wiring in PCB, particularlyat plated through holes and high aspect ratio blind vias. Corrosion atthese locations manifests itself as an annular ring surrounding the viasand plated through holes. Some exposed bare copper is present at theedge of soldermask. Additionally, immersion silver is beset by intrinsicpore formation. In other words, immersion silver processes, beingself-limiting, deposit relatively thin layers. These thin layers areporous. Finally, silver is susceptible to sulfidation by reduced sulfurcompounds (e.g., hydrogen sulfide) present in the environment,particularly at paper processing plants, rubber processing plants, andhigh pollution environments.

Sufficient sulfidation of silver can result in localized areas of silversulfide salts that, if they grow large enough, may separate from thesilver layer, also forming pores. Exposed areas of copper, which mayresult from insufficient coverage from the immersion plating process,from intrinsic pores in the layer from the immersion silver process, orfrom later formed pores caused by sulfidation, are susceptible to creepcorrosion. Humidity and environmental pollutants can oxidize andsulfidize the copper, forming copper salts that may creep through anylocation of insufficient copper coverage by the immersion silver layer.

Immersion silver coatings have been protected with a coating comprisinga mercaptan. Mercaptans, however, may not sufficiently protect the boardfrom creep corrosion. Moreover, mercaptan coatings may degrade duringassembly processes employing lead-free solders, which typically occur attemperatures above 220° C. and may be as high as 270° C.

SUMMARY OF THE INVENTION

Briefly, therefore, the invention is directed to a composition forenhancing the corrosion resistance of an immersion-plated silver coatingdeposited on a solderable copper substrate, the composition comprising amulti-functional molecule, wherein the multi-functional moleculescomprises at least one nitrogen-containing organic functional group thatinteracts with and protects copper surfaces and at least onesulfur-containing organic functional group that interacts with andprotects silver surfaces; an alcohol; a surfactant; and an alkaline pHadjuster.

The invention is also directed to a method of enhancing the corrosionresistance of an article comprising a silver coating deposited on asolderable copper substrate, the method comprising exposing the coppersubstrate having the immersion-plated silver coating thereon to ananti-corrosion composition comprising a) a multi-functional molecule,wherein the multi-functional molecules comprises at least one organicfunctional group that interacts with and protects copper surfaces and atleast one organic functional group that interacts with and protectssilver surfaces; b) an alcohol; and c) a surfactant.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a depiction of a copper substrate having an immersion silvercoating deposited thereon further covered with a protective organic filmof the present invention.

FIG. 2 is a photograph displaying panels subjected to corrosion testingaccording to the method of Example 4.

FIG. 3 is a photograph displaying coupons subjected to corrosion testingaccording to the method described in Example 5.

FIG. 4 is a graph showing the results of wetting balance evaluationaccording to the method described in Example 6.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE EMBODIMENT(S) OF THE INVENTION

This application claims priority from U.S. provisional application60/986,481 filed Nov. 8, 2008, the entire disclosure of which isincorporated by reference.

The present invention is directed to a method and composition forapplying a protective organic film to a silver coating. The silvercoating may be applied by an immersion silver plating method or by anelectrolytic plating method. In a preferred embodiment, the silvercoating is plated by an immersion silver plating method over a coppersubstrate. The protective organic film is particularly suited forpreserving the integrity of the immersion silver finish and theunderlying copper substrate, thereby resulting in, e.g., improvedappearance, corrosion resistance, creep corrosion resistance, andsolderability of copper or copper alloy substrates having a layer ofimmersion-plated silver thereon. Copper substrates suitable forprotection with the organic protective film of the invention includecircuit boards, chip carriers, semiconductor substrates, metal leadframes, connectors, and other solderable copper substrates. Silverimmersion displacement plating is one method of preserving thesolderability of these copper substrates. Silver immersion plating is aself-limiting process which yields silver layers having typicalthicknesses between about 0.05 microns and about 0.8 microns, typicallybetween about 0.15 microns and about 0.40 microns. Certain immersionprocesses and compositions can plate silver layers having thicknessesoutside the broad range.

As stated above, immersion-plated silver may not adequately protectcopper surfaces, such as at certain bare copper interfaces betweencopper and silver, particularly at plated through holes and high aspectratio blind vias in PCB substrates. Moreover, immersion-plated silvercoatings are characterized by intrinsic pores due to the self-limitingnature of the process. Finally, immersion-plated silver surfaces aresusceptible to pore formation due to plating processes in addition tosulfidation and oxidation, particularly in high pollution environments.Accordingly, the present invention is directed to a method of applying aprotective organic film to provide a layer of corrosion protection overcopper surfaces, in addition to the immersion-plated silver coating. Themethod of applying the protective organic film involves exposing thecopper substrate having a silver coating on a surface thereof to acomposition for enhancing the corrosion resistance of animmersion-plated silver coating deposited on a solderable coppersubstrate.

The present invention is therefore further directed to such acomposition. The composition comprises a molecule comprising functionalgroups capable of interacting with and protecting copper and silversurfaces. In one embodiment, the molecule comprises two or morefunctional groups with distinct functionality, i.e., a multi-functionalmolecule. Multi-functional molecules encompass bi-functional moleculesin which the molecules comprise two organic functional groups withdistinct functionality. According to the present invention, thebi-functional molecule comprises at least one organic functional groupwhich interacts with and protects copper surfaces and at least oneorganic functional group which interacts with and protects silversurfaces. Multi-functional molecules, in the context of the presentinvention, further encompass tri-functional molecules, tetra-functionalmolecules, and so on, each molecule having three, four, or more organicfunctional groups with distinct functionality. In one embodiment, theorganic protective film may be characterized as a self-assembledmonolayer comprising the multi-functional molecule.

The multi-functional molecule comprises at least one organic functionalgroup that interacts with and protects copper surfaces. In oneembodiment, the organic functional group that interacts with andprotects copper surfaces is an amine. An amine is a functional groupcomprising nitrogen, typically bonded to an organic substituent, such asa hydrocarbyl or an aryl. Hydrocarbyl encompasses alkyl, alkenyl, andalkynyl. The hydrocarbyl may be substituted or unsubstituted. Arylencompasses aromatic groups, such as phenyl, naphthenyl, and groupshaving more than two fused rings. The aryl may be substituted orunsubstituted and may be homocyclic or heterocyclic.

Applicable amines include primary amines, secondary amines, tertiaryamines, and aromatic heterocycles comprising nitrogen. Primary amines,secondary amines, and tertiary amines may have the general structure(I):

wherein R₁, R₂, and R₃ are either hydrocarbyl, aryl, or hydrogen, and atleast one of R₁, R₂, and R₃ is hydrocarbyl or aryl. In a typicalstructure, at least one of R₁, R₂, and R₃ is a carbon chain of thehydrocarbyl comprising between about two and about 24 carbon atoms,typically between about six and about 24 carbon atoms, more typicallybetween about 10 and about 18 carbon atoms. Aryl groups typicallybetween about six and about 24 carbon atoms, more typically betweenabout six and about 10 carbon atoms, i.e., a phenyl group (substitutedbenzene), a naphthenyl groups (substituted naphthalene), a substitutedanthracene, a substituted phenanthrene, a substituted tetracene, and soon. The hydrocarbyl and the aryl may be further substituted. Typicalsubstituents include short carbon chain branching alkyl groups,typically having from one to four carbon atoms, i.e., methyl, ethyl,propyl, and butyl substituents and aromatic groups such as phenyl,naphthenyl, and aromatic heterocycles comprising nitrogen, oxygen, andsulfur. Other substituents include additional amines, thiols,carboxylates, phosphates, phosphonates, sulfates, sulfonates, halogen,hydroxyl, alkoxy, aryloxy, protected hydroxy, keto, acyl, acyloxy,nitro, cyano, esters, and ethers.

The aromatic heterocycle comprising nitrogen is preferably a 5-memberedaromatic ring (an azole). The ring can be substituted at a carbon atom,a nitrogen atom, or both. Preferably, the ring is substituted at acarbon atom. The substituent may be the organic functional group capableof interacting with and protecting silver surfaces. Other applicablesubstituents include short carbon chain alkyl groups, typically havingfrom one to four carbon atoms, i.e., methyl, ethyl, propyl, and butylsubstituents and aromatic groups such as phenyl, naphthenyl, andaromatic heterocycles comprising nitrogen, oxygen, and sulfur. Othersubstituents include amines, thiols, carboxylates, phosphates,phosphonates, sulfates, sulfonates, halogen, hydroxyl, alkoxy, aryloxy,protected hydroxy, keto, acyl, acyloxy, nitro, cyano, esters, andethers. The ring can be fused to aromatic or cycloalkyl groups, whichmay be homocyclic or heterocyclic. In one embodiment, the ring is fusedto a 6-membered ring. Exemplary azoles which can be further substitutedwith additional functional groups are shown in Tables 1.

TABLE 1 Azoles Name Structure Pyrrole (1H-azole)

Imidazole (1,3-diazole)

Pyrazole (1,2-diazole)

1,2,3-triazole

1,2,4-triazole

Tetrazole

Isoindole

Indole (1H- Benzo[b]pyrrole)

Benzimidazole (1,3- benzodiazole)

Indazole (1,2- benzodiazole)

1H- Benzotriazole

2H- Benzotriazole

Imidazo[4,5- b]pyridine

Purine (7H- imidazo(4,5- d) pyrimidine)

Pyrazolo[3,4- d]pyrimidine

Triazolo[4,5- d]pyrimidine

Preferred aromatic heterocyclic compounds comprising nitrogen includeimidazole, triazole, pyrazole, benzimidazole, purine,imidazo[4,5-b]pyridine, and benzotriazole. Among these, benzimidazole isparticularly preferred.

Without being bound to a particular theory, it is thought that primaryamines, secondary amines, tertiary amines, and aromatic heterocyclescomprising nitrogen interact with copper(I) ions on the surface of thecopper conducting layer and copper(II) ions in solution. Interactionwith copper(I) ions forms a film comprising insoluble copper(I)-basedorganometallics on the surface of the copper conducting layer, i.e., afilm that has become known as an organometallic conversion coating(OMCC). The aromatic heterocycle comprising nitrogen chelates copper(II)ions in solution. These interactions result in the formation of aprotective film on the surface of the copper conductive layer which isenriched in copper(I) ions, thereby increasing the ratio of copper(I)ions to copper(II) ions on the surface of the copper conducting layer.It is further thought that primary amines, secondary amines, tertiaryamines, and aromatic heterocycles comprising nitrogen formnitrogen-copper bonds on surfaces of the copper substrate and also mayform nitrogen-silver bonds on surfaces of the silver layer. Bondingrepresents an additional means by which organic functional groupscomprising nitrogen form an organic protective layer over the copper andsilver surfaces.

The multi-functional molecule comprises at least one organic functionalgroup that interacts with and protects silver surfaces. In oneembodiment, the organic functional group that interacts with andprotects silver surfaces comprises sulfur. Organic functional groupscomprising sulfur include thiol, disulfide, thioether, thioaldehyde,thioketone, and aromatic heterocycles comprising sulfur. Preferredorganic functional groups comprising sulfur are thiol and disulfide. Athiol is a functional group comprising a sulfur atom bonded to ahydrogen atom and an organic substituent, such as a hydrocarbyl or anaryl. A disulfide is a functional group comprising a sulfur atom bondedto another sulfur atom and an organic substituent, such as a hydrocarbylor an aryl. The hydrocarbyl may comprise between about two and about 24carbon atoms, typically between about six and about 24 carbon atoms,more typically between about 10 and about 18 carbon atoms. Aryl groupstypically between about six and about 24 carbon atoms, more typicallybetween about six and about 10 carbon atoms, i.e., phenyl, andnaphthenyl groups. The hydrocarbyl and the aryl may be substituted orunsubstituted. Typical substituents include short carbon chain branchingalkyl groups, typically having from one to four carbon atoms, i.e.,methyl, ethyl, propyl, and butyl substituents and aromatic groups suchas phenyl, naphthenyl, and aromatic heterocycles comprising nitrogen,oxygen, and sulfur. Other substituents include amines, thiols,carboxylates, phosphates, phosphonates, sulfates, sulfonates, halogen,hydroxyl, alkoxy, aryloxy, protected hydroxy, keto, acyl, acyloxy,nitro, cyano, esters, and ethers.

It has been discovered that organic functional groups comprising sulfurprimarily form sulfur-silver bonds on surfaces of the silver layer. Theymay also form sulfur-copper bonds on surfaces of the copper substrate.

According to the present invention, the organic functional group thatinteracts with and protects copper surfaces and the organic functionalgroup that interacts with and protects silver surfaces are located onthe same molecule, thus making the molecule a multi-functional molecule.Stated another way, the multi-functional molecule comprises a functionalgroup comprising nitrogen and a functional group comprising sulfur.

In one embodiment, the multi-functional molecule comprises a functionalgroup comprising nitrogen and a thiol. The multi-functional molecule maycomprise additional functionality, and typically comprises hydrocarbylor aryl that links the organic functional groups together. For example,the multi-functional molecule may comprise a hydrocarbyl group thatlinks the amine and the thiol through a carbon chain and have a generalstructure (II):

wherein R₁ is hydrocarbyl and R₂ and R₃ are hydrocarbyl, nitrogen, orhydrogen. The carbon chain of the hydrocarbyl may comprise between abouttwo and about 24 carbon atoms, typically between about six and about 24carbon atoms, more typically between about 12 and about 18 carbon atoms.The carbon chain of the hydrocarbyl may be substituted or unsubstituted.Typical substituents include short carbon chain branching alkyl groups,typically having from one to four carbon atoms, i.e., methyl, ethyl,propyl, and butyl substituents and aromatic groups such as phenyl,naphthenyl, and aromatic heterocycles comprising nitrogen, oxygen, andsulfur. Other substituents include amines, thiols, carboxylates,phosphates, phosphonates, sulfates, sulfonates, halogen, hydroxyl,alkoxy, aryloxy, protected hydroxy, keto, acyl, acyloxy, nitro, cyano,esters, and ethers. In one preferred embodiment, the R₁ hydrocarbyl isnot substituted with other groups, as straight-chained hydrocarbonsbetter achieve desirable densely packed self-assembled monolayer on thesilver and copper surfaces.

In one embodiment, the multi-functional molecule defined by structure(II) comprises an amine and a thiol. The amine may be a primary amine, asecondary amine, or a tertiary amine. Exemplary multi-functionalmolecules comprising an amine and a thiol include cysteine, methionine,2-Aminoethanethiol (cysteamine), 3-aminopropanethiol,4-aminobutanethiole, 5-aminopentanethiol, 6-aminohexanethiol,8-aminooctanethiol, 10-aminodecanethiol, and 12-aminododecanethiol. Themulti-functional groups comprising relatively long chain hydrocarbonsmay have the amino functionality at locations other than the oppositeend of the hydrocarbon chain from the thiol group. For example,applicable aminododecanethiols include those in which the aminofunctional group is located at any of the carbons in the hydrocarbonchain.

In one embodiment, the multi-functional molecule defined by structure(II) comprises an aromatic heterocycle comprising nitrogen and a thiol.In one embodiment, the nitrogen atom, R₂, and R₃ of structure (II) formsa 5-membered aromatic heterocyclic ring. The other two atoms in the5-membered ring may be carbon atoms or nitrogen atoms. The 5-memberedaromatic heterocyclic ring may be unfused (i.e., a pyrrole, animidazole, a pyrazole, a triazole, or a tetrazole) or may be fused to asix-member ring (i.e., an isoindole, an indole, a benzimidazole, anindazole, a benzotriazole, a purine, or an imidazo[4,5-b]pyridine). SeeTable I above. In this embodiment, the multi-functional molecule has thestructure (IIa):

wherein R₁ is hydrocarbyl and R₂, R₃, R₄, R₅ are nitrogen, sulfur, orcarbon. The carbon chain of the hydrocarbyl may comprise between abouttwo and about 24 carbon atoms, typically between about six and about 24carbon atoms, more typically between about 12 and about 18 carbon atoms.Any of the carbon chain of the hydrocarbyl, R₂, R₃, R₄, and R₅ may besubstituted or unsubstituted. Typical substituents include short carbonchain branching alkyl groups, typically having from one to four carbonatoms, i.e., methyl, ethyl, propyl, and butyl substituents and aromaticgroups such as phenyl, naphthenyl, and aromatic heterocycles comprisingnitrogen, oxygen, and sulfur. Other substituents include amines, thiols,carboxylates, phosphates, phosphonates, sulfates, sulfonates, halogen,hydroxyl, alkoxy, aryloxy, protected hydroxy, keto, acyl, acyloxy,nitro, cyano, esters, and ethers. In one preferred embodiment, the R₁hydrocarbyl is not substituted with other groups, as straight-chainedhydrocarbons better achieve desirable densely packed self-assembledmonolayer on the silver and copper surfaces.

In one embodiment, the nitrogen atom, a carbon atom from R₁, R₂, and R₃of structure (II) forms a 5-membered aromatic heterocyclic ring. Theother atom in the 5-membered ring may be carbon atom or nitrogen atom.The 5-membered aromatic heterocyclic ring may be unfused (i.e., apyrrole, an imidazole, a pyrazole, a triazole, or a tetrazole) or may befused to a six-member ring (i.e., an isoindole, an indole, abenzimidazole, an indazole, a benzotriazole, a purine, or animidazo[4,5-b]pyridine). See Table I above. In this embodiment, themulti-functional molecule may have any of the general structures (IIb)through (IIe):

wherein R₁ is hydrocarbyl and R₂, R₃, and R₄ are nitrogen, sulfur, orcarbon. The carbon chain of the hydrocarbyl may comprise between abouttwo and about 24 carbon atoms, typically between about six and about 24carbon atoms, more typically between about 12 and about 18 carbon atoms.Any of the carbon chain of the hydrocarbyl, R₂, R₃, and R₄ may besubstituted or unsubstituted. Typical substituents include short carbonchain branching alkyl groups, typically having from one to four carbonatoms, i.e., methyl, ethyl, propyl, and butyl substituents and aromaticgroups such as phenyl, naphthenyl, and aromatic heterocycles comprisingnitrogen, oxygen, and sulfur. Other substituents include amines, thiols,carboxylates, phosphates, phosphonates, sulfates, sulfonates, halogen,hydroxyl, alkoxy, aryloxy, protected hydroxy, keto, acyl, acyloxy,nitro, cyano, esters, and ethers. In one preferred embodiment, the R₁hydrocarbyl is not substituted with other groups, as straight-chainedhydrocarbons better achieve desirable densely packed self-assembledmonolayer on the silver and copper surfaces.

Exemplary multi-functional molecules comprising an aromatic heterocyclecomprising nitrogen and a thiol for use in the anti-corrosioncomposition and for use in a protective film over immersion silver andcopper surfaces include:

-   2-mercaptobenzimidazole;-   2-mercapto-5-methylbenzimidazole;-   2-mercapto-5-nitrobenzimidazole;-   5-Amino-2-mercaptobenzimidazole;-   5-Ethoxy-2-mercaptobenzimidazole;-   5-(difluoromethoxy)-2-mercapto-1H-benzimidazole;-   2-mercapto-1-methylimidazole;-   1-Methyl-1H-benzimidazole-2-thiol;-   1-[2-(Dimethylamino)ethyl]-1H-tetrazole-5-thiol,    1-(4-Hydroxyphenyl)-1H-tetrazole-5-thiol;-   1-(2-methoxyphenyl)-4-(4-nitrophenyl)-1H-imidazole-2-thiol;-   1-(2-methylphenyl)-4-(4-methylphenyl)-1H-imidazole-2-thiol;-   4-Phenylthiazole-2-thiol;-   1H-1,2,4-Triazole-3-thiol;-   2-Thiazoline-2-thiol;-   4-Amino-6-mercaptopyrazolo[3,4-d]pyrimidine;-   3-Amino-1,2,4-triazole-5-thiol;-   4-Amino-5-(4-pyridyl)-4H-1,2,4-triazole-3-thiol;-   4-Amino-5-phenyl-4H-1,2,4-triazole-3-thiol;-   5-amino-1,3,4-thiadiazole-2-thiol;-   2-mercapto-5-methylamino-1,3,4-thiadiazole;-   5-mercapto-1-methyltetrazole;-   1-phenyl-1H-tetrazole-5-thiol; and

other bath-compatible molecules having an azole and thiol functionalgroups.

In one embodiment, the multi-functional molecule comprises a functionalgroup comprising nitrogen and a disulfide. This multi-functionalmolecule is substantially similar to the molecule comprising afunctional group comprising nitrogen and the thiol, except that twothiols are bonded together through a disulfide linkage, —S—S—.Accordingly, the multi-functional molecule may have the followinggeneral structure (III):

wherein R₁ and R₄ are hydrocarbyl and R₂, R₃, R₅, and R₆ arehydrocarbyl, nitrogen, or hydrogen. The carbon chain of the hydrocarbylmay comprise between about two and about 24 carbon atoms, typicallybetween about six and about 24 carbon atoms, more typically betweenabout 12 and about 18 carbon atoms. The carbon chain of the hydrocarbylmay be substituted or unsubstituted. Typical substituents include shortcarbon chain branching alkyl groups, typically having from one to fourcarbon atoms, i.e., methyl, ethyl, propyl, and butyl substituents andaromatic groups such as phenyl, naphthenyl, and aromatic heterocyclescomprising nitrogen, oxygen, and sulfur. Other substituents includeamines, thiols, carboxylates, phosphates, phosphonates, sulfates,sulfonates, halogen, hydroxyl, alkoxy, aryloxy, protected hydroxy, keto,acyl, acyloxy, nitro, cyano, esters, and ethers. In one preferredembodiment, the R₁ hydrocarbyl is not substituted with other groups, asstraight-chained hydrocarbons better achieve desirable densely packedself-assembled monolayer on the silver and copper surfaces. Exemplarymolecules comprising a functional group comprising nitrogen and adisulfide include 2,2′-Dipyridyl disulfide, 4,4′-Dipyridyl disulfide2-aminophenyl disulfide, 4-aminophenyl disulfide, cystamine (commonlyavailable as the dihydrochloride salt), bis(2-aminoethyl) disulfide,bis(3-aminopropyl) disulfide, bis(4-aminobutyl) disulfide,bis(5-aminopentyl) disulfide, bis(6-aminohexyl) disulfide,bis(7-aminoheptyl) disulfide, bis(8-aminooctyl) disulfide,bis(10-aminodecyl) disulfide, and disulfides with longer carbon chains.

The multi-functional molecule may be present in the anti-corrosioncomposition at a typical concentration of about 3 g/L. The concentrationis typically at this minimum concentration to achieve adequate coverageof the substrate for corrosion protection. Typically, the concentrationof the multi-functional molecule is at least about 0.01 g/L, moretypically at least about 0.1 g/L, even more typically at least about 1g/L. The multi-functional molecule may be present in the anti-corrosioncomposition at a concentration up to its solubility limit, typically atmost about 100 g/L. Typically, the concentration of the multi-functionalmolecule is less than about 10 g/L, more typically less than about 6g/L. Accordingly, the concentration of the multi-functional molecule maybe between about 0.1 g/L and about 10 g/L, typically between about 1 g/Land about 6 g/L, such as in one embodiment about 3 g/L.

The anti-corrosion composition is preferably an aqueous solutioncomprising a multi-functional molecule as described above. Theanti-corrosion composition of the present invention may further comprisean alcohol, a surfactant, and an alkaline pH adjuster.

Incorporating an alcohol in the anti-corrosion composition enhances thesolubility of the multi-functional compound. Applicable alcohols includealcohols, diols, triols, and higher polyols. Suitable alcohols includeethanol, propanol, isopropanol, n-butanol, isobutanol, tert-butanol,ethylene glycol, propane-1,2-diol, butane-1,2-diol, butane-1,3-diol,butane-1,4-diol, propane-1,3-diol, hexane-1,4-diol hexane-1,5-diol,hexane-1,6-diol, etc. Then there are unsaturated diols, such asbutene-diol, hexene-diol, and acetylenics such as butyne diol. Asuitable triol is glycerol. Additional alcohols include triethyleneglycol, diethylene glycol, diethylene glycol methyl ether, triethyleneglycol monomethyl ether, triethylene glycol dimethyl ether, propyleneglycol, dipropylene glycol, allyl alcohol, furfuryl alcohol, andtetrahydrofurfuryl alcohol.

The alcohol may be present in the anti-corrosion composition at aconcentration of at least about 10 mL/L. Typically, the concentration ofthe alcohol is at least about 100 mL/L, more typically at least about150 mL/L. The alcohol may be present in the anti-corrosion compositionat a concentration up to its solubility limit in water. It is within thescope of the invention to employ solvent systems comprised entirely ofalcohol. In aqueous solvent systems wherein the alcohol is asupplementary solvent, the concentration of the alcohol is typicallyless than about 500 mL/L, more typically less than about 200 mL/L.Accordingly, the alcohol concentration may be between about 10 mL/L andabout 500 mL/L, typically between about 150 mL/L and about 200 mL/L.

A surfactant may be added to enhance the wettability of the copper andsilver surfaces. The surfactant may be cationic, anionic, non-ionic, orzwitterionic. A particular surfactant may be used alone or incombination with other surfactants. One class of surfactants comprises ahydrophilic head group and a hydrophobic tail. Hydrophilic head groupsassociated with anionic surfactants include carboxylate, sulfonate,sulfate, phosphate, and phosphonate. Hydrophilic head groups associatedwith cationic surfactants include quaternary amine, sulfonium, andphosphonium. Quaternary amines include quaternary ammonium, pyridinium,bipyridinium, and imidazolium. Hydrophilic head groups associated withnon-ionic surfactants include alcohol and amide. Hydrophilic head groupsassociated with zwitterionic surfactants include betaine. Thehydrophobic tail typically comprises a hydrocarbon chain. Thehydrocarbon chain typically comprises between about six and about 24carbon atoms, more typically between about eight to about 16 carbonatoms.

Exemplary anionic surfactants include alkyl phosphonates, alkyl etherphosphates, alkyl sulfates, alkyl ether sulfates, alkyl sulfonates,alkyl ether sulfonates, carboxylic acid ethers, carboxylic acid esters,alkyl aryl sulfonates, and sulfosuccinates. Anionic surfactants includeany sulfate ester, such as those sold under the trade name ULTRAFAX,including, sodium lauryl sulfate, sodium laureth sulfate (2 EO), sodiumlaureth, sodium laureth sulfate (3 EO), ammonium lauryl sulfate,ammonium laureth sulfate, TEA-lauryl sulfate, TEA-laureth sulfate,MEA-lauryl sulfate, MEA-laureth sulfate, potassium lauryl sulfate,potassium laureth sulfate, sodium decyl sulfate, sodium octyl/decylsulfate, sodium 2-ethylhexyl sulfate, sodium octyl sulfate, sodiumnonoxynol-4 sulfate, sodium nonoxynol-6 sulfate, sodium cumene sulfate,and ammonoium nonoxynol-6 sulfate; sulfonate esters such as sodiumα-olefin sulfonate, ammonium xylene sulfonate, sodium xylene sulfonate,sodium toluene sulfonate, dodecyl benzene sulfonate, andlignosulfonates; sulfosuccinate surfactants such as disodium laurylsulfosuccinate, disodium laureth sulfosuccinate; and others includingsodium cocoyl isethionate, lauryl phosphate, any of the ULTRAPHOS seriesof phosphate esters, Cyastat® 609(N,N-Bis(2-hydroxyethyl)-N-(3′-Dodecyloxy-2′-Hydroxypropyl) MethylAmmonium Methosulfate) and Cyastat® LS ((3-Lauramidopropyl)trimethylammonium methylsulfate), available from Cytec Industries.

Exemplary cationic surfactants include quaternary ammonium salts such asdodecyl trimethyl ammonium chloride, cetyl trimethyl ammonium salts ofbromide and chloride, hexadecyl trimethyl ammonium salts of bromide andchloride, alkyl dimethyl benzyl ammonium salts of chloride and bromide,and the like. In this regard, surfactants such as Lodyne 106A(Fluoroalkyl Ammonium Chloride Cationic Surfactant 28-30%) and Ammonyx4002 (Octadecyl dimethyl benzyl ammonium chloride Cationic Surfactant)are particularly preferred.

In a preferred embodiment, the surfactant is non-ionic. A class ofnon-ionic surfactants includes those comprising polyether groups, basedon, for example, ethylene oxide (EO) repeat units and/or propylene oxide(PO) repeat units. These surfactants are typically non-ionic.Surfactants having a polyether chain may comprise between about 1 andabout 36 EO repeat units, between about 1 and about 36 PO repeat units,or a combination of between about 1 and about 36 EO repeat units and POrepeat units. More typically, the polyether chain comprises betweenabout 2 and about 24 EO repeat units, between about 2 and about 24 POrepeat units, or a combination of between about 2 and about 24 EO repeatunits and PO repeat units. Even more typically, the polyether chaincomprises between about 6 and about 15 EO repeat units, between about 6and about 15 PO repeat units, or a combination of between about 6 andabout 15 EO repeat units and PO repeat units. These surfactants maycomprise blocks of EO repeat units and PO repeat units, for example, ablock of EO repeat units encompassed by two blocks of PO repeat units ora block of PO repeat units encompassed by two blocks of EO repeat units.Another class of polyether surfactants comprises alternating PO and EOrepeat units. Within these classes of surfactants are the polyethyleneglycols, polypropylene glycols, and the polypropyleneglycol/polyethylene glycols.

Yet another class of non-ionic surfactants comprises EO, PO, or EO/POrepeat units built upon an alcohol or phenol base group, such asglycerol ethers, butanol ethers, pentanol ethers, hexanol ethers,heptanol ethers, octanol ethers, nonanol ethers, decanol ethers,dodecanol ethers, tetradecanol ethers, phenol ethers, alkyl substitutedphenol ethers, α-naphthol ethers, and β-naphthol ethers. With regard tothe alkyl substituted phenol ethers, the phenol group is substitutedwith a hydrocarbon chain having between about 1 and about 10 carbonatoms, such as about 8 (octylphenol) or about 9 carbon atoms(nonylphenol). The polyether chain may comprise between about 1 andabout 24 EO repeat units, between about 1 and about 24 PO repeat units,or a combination of between about 1 and about 24 EO and PO repeat units.More typically, the polyether chain comprises between about 8 and about16 EO repeat units, between about 8 and about 16 PO repeat units, or acombination of between about 8 and about 16 EO and PO repeat units. Evenmore typically, the polyether chain comprises about 9, about 10, about11, or about 12 EO repeat units; about 9, about 10, about 11, or about12 PO repeat units; or a combination of about 9, about 10, about 11, orabout 12 EO repeat units and PO repeat units.

An exemplary β-naphthol derivative non-ionic surfactant is LugalvanBNO12 which is a β-naphtholethoxylate having 12 ethylene oxide monomerunits bonded to the naphthol hydroxyl group. A similar surfactant isPolymax NPA-15, which is a polyethoxylated nonylphenol. Anothersurfactant is Triton®-X100 nonionic surfactant, which is an octylphenolethoxylate, typically having around 9 or 10 EO repeat units. Additionalcommercially available non-ionic surfactants include the Pluronic®series of surfactants, available from BASF. Pluronic® surfactantsinclude the P series of EO/PO block copolymers, including P65, P84, P85,P103, P104, P105, and P123, available from BASF; the F series of EO/POblock copolymers, including F108, F127, F38, F68, F77, F87, F88, F98,available from BASF; and the L series of EO/PO block copolymers,including L10, L101, L121, L31, L35, L44, L61, L62, L64, L81, and L92,available from BASF.

Additional commercially available non-ionic surfactants include watersoluble, ethoxylated nonionic fluorosurfactants available from DuPontand sold under the trade name Zonyl®, including Zonyl® FSN (Telomar BMonoether with Polyethylene Glycol nonionic surfactant), Zonyl® FSN-100,Zonyl® FS-300, Zonyl® FS-500, Zonyl® FS-510, Zonyl® FS-610, Zonyl® FSP,and Zonyl® UR. Other non-ionic surfactants include the aminecondensates, such as cocoamide DEA and cocoamide MEA, sold under thetrade name ULTRAFAX. Other classes of nonionic surfactants include acidethoxylated fatty acids (polyethoxy-esters) comprising a fatty acidesterified with a polyether group typically comprising between about 1and about 36 EO repeat units. Glycerol esters comprise one, two, orthree fatty acid groups on a glycerol base.

The surfactant may be present in the preferred anti-corrosioncomposition at a concentration of at least about 0.01 g/L. Manysurfactants provide effective wetting at very low concentrations. Theminimum concentration may be adjusted to achieve adequate wetting, whichdepends in part on the identity of the surfactant. Typically, thesurfactant concentration is at least about 0.1 g/L, more typically atleast about 0.5 g/L. The surfactant may be present in the anti-corrosioncomposition at a concentration of less than about 10.0 g/L. Typically,the surfactant concentration is less than about 5.0 g/L, more typicallyless than about 2.0 g/L.

The anti-corrosion composition of the present invention preferably has apH between about 1.0 and about 12.0, typically between about 7.0 andabout 11.0. The composition is preferably alkaline because in alkalinesolution, the formation of the protective organic coating is more rapidthan its formation in acidic solution. Alkaline adjustment may beaccomplished using alkaline pH adjusting agents, such as sodiumhydroxide, potassium hydroxide, hydroxides of quaternary amines, such astetramethylammonium hydroxide, tetraethylammonium hydroxide, and thelike. Typically, the concentration of the alkaline pH adjuster issufficient to achieve the desired alkaline pH and may be between about0.01 g/L and about 10.0 g/L, typically between about 0.01 g/L and about2.0 g/L, more typically between about 0.1 g/L and about 0.5 g/L.

In certain particularly preferred embodiments, the composition is freeof alkali metal hydroxide, or specifically free of sodium hydroxide, andonly an alternative agent such as sodium tetra borate is used for pHadjustment.

In an optional embodiment, some metal ions, such as zinc and copper ion,can be incorporated into this formula to help building a thicker film,which results in better corrosion resistance, thermal resistance, andwear resistance.

Another aspect of the present invention is directed to a method ofenhancing the corrosion resistance of an immersion-plated silver coatingdeposited on a solderable copper substrate. The method involves exposinga copper substrate having an immersion-plated silver coating thereon toan anti-corrosion composition comprising a multi-functional molecule.

The copper substrate may be prepared with an immersion-plated silvercoating by methods known in the art. For example, the method of coatinga copper substrate with immersion-plated silver described in U.S. Pub.No. 2006/0024430, herein incorporated by reference in its entirety, isapplicable. Commercially available chemistries for immersion silvercoating include AlphaSTAR®, available from Enthone Inc. (West Haven,Conn.). Prior to exposure to the anti-corrosion composition, it may beadvantageous in certain embodiments to etch the immersion-plated silvercoating conventional etchants. It may be advantageous in certainembodiments to rinse the immersion-plated silver coating in alkaline oracidic solutions.

In an alternative embodiment, the substrate may be plated with anelectrolytic silver coating by methods known in the art. For example,the electrolytic silver coating may be plated using SILVREX® White MetalTechnology, available from Enthone Inc., West Haven, Conn., usingconditions suggested by the manufacturer. This process employs analkaline cyanide silver plating bath with a silver ion concentrationbetween about 20 g/L and about 50 g/L and a pH between about 11 andabout 13. The current density may vary between about 0.5 A/dm² and about3 A/dm² at a plating temperature between about 15° C. and about 45° C.,such that the time to plate a 1 micron thick layer is between about 1and about 3 minutes. Silver purities exceeding 99% may be achieved usingthe SILVREX® plating chemistry.

The anti-corrosion composition comprising a multi-functional moleculemay be applied to the substrate in any manner sufficient to achieveadequate coverage of the substrate surface. By adequate, it is meantthat the method of exposure ensures that areas of bare copper arecovered with the anti-corrosion composition, particularly copper-silverinterfaces at high aspect ratio blind vias and plated through holes andpores that may be present in the immersion silver coating. Adequatecoverage ensures that the multi-functional molecule can interact withbare copper surfaces and silver surfaces in a manner sufficient to forma protective organic film over the copper and silver surfaces. Exposuremay be by flooding, dip, cascade, or spraying. The duration of exposureis not narrowly critical to the efficacy of the invention and may dependin part on engineering aspects of the process. Typical exposure timesmay be as little as about 1 second to as long as about 20 minutes, suchas between about 1 second and about 10 minutes. In practice, theexposure time may be between about 15 seconds and about 120 seconds,typically between about 15 seconds and about 60 seconds, such as betweenabout 30 seconds and about 60 seconds. In view of these relatively shortexposure times, the method of the present invention achieves rapidsubstrate coating. The temperature of the anti-corrosion composition mayvary between room temperature up to about 75° C., typically betweenabout 25° C. and about 55° C., such as between about 25° C. and about45° C. Exposure of bare copper areas to the anti-corrosion coating maybe enhanced with scrubbing, brushing, squeegeeing, agitation, andstirring. In particular, agitation has been shown to be an effectivemeans on enhancing the ability of the composition to apply a protectiveorganic coating to the substrate. After exposing the copper substrate tothe anti-corrosion composition, the substrate may be rinsed, typicallywith deionized water for between about 10 seconds to about 2 minutes.

Another aspect of the present invention is directed to a protectiveorganic film applied over an immersion silver coating deposited on asolderable copper substrate. Exposure of the copper substrate having animmersion silver coating thereon to the anti-corrosion composition ofthe present invention results in a protective organic film on both thesilver surfaces and exposed copper surfaces. The protective organic filmcomprises the multi-functional molecule wherein the organic functionalgroup comprising nitrogen interacts with and protects exposed coppersurfaces and the organic functional group comprising sulfur interactswith and protects silver surfaces. A depiction of this protectiveorganic film is shown in FIG. 1, in which the functional groups of themulti-functional molecules that constitute the protective organic film 2are shown interacting with both the copper substrate 4 and the immersionsilver coating 6.

The multi-functional molecule interacts with the copper and silversurfaces in a manner similar to the conventional self-assembly oforganic thiols adsorbed to gold surfaces. Accordingly, themulti-functional molecules self-assemble into a monolayer on the copperand silver surfaces. The thickness of the film may vary fromapproximately the length of the multi-functional molecule to severalmultiples (i.e., 3 or 4 times) the length of the multi-functionalmolecule. Typical film thicknesses may be between about 50 Angstroms andabout 1 micron, more typically between about 50 Angstroms and about 1000Angstroms. Accordingly, the protective organic film is a relativelydense, hydrophobic film that can provide enhanced protection againstatmospheric moisture, which in turn, enhances the immersion silvercoating's resistance to corrosion and sulfidation.

The protective organic film of the present invention may be additionallycharacterized by high thermal stability, particularly to temperaturescommonly reached during lead-free reflow. The protective organiccoatings of the present invention can better withstand reflowtemperatures compared to conventional organic coatings (such as OSP) asshown by differential scanning calorimetry and thermogravimetricanalysis (TGA). For example, a protective organic coating comprising2-mercaptobenzimidazole is stable at temperatures as high as about 254°C., while only 5% of the film is lost at temperatures as high as 274° C.This compares favorably to typical reflow temperatures for eutectictin-lead solder, which is typically reflowed at temperatures betweenabout 230° C. and about 235° C. Moreover, the protective organic coatingcan withstand multiple lead-free reflow processes.

Additionally, the protective organic coating has been observed to notnegatively impact visual appearance and the solderability of the coppersubstrate. Solderability is shown by wetting balance testing and contactresistance. Finally, the protective organic coating has been observed toincrease the wear resistance and lubricity of an immersion-plated silvercoating.

Having described the invention in detail, it will be apparent thatmodifications and variations are possible without departing from thescope of the invention defined in the appended claims.

EXAMPLES

The following non-limiting examples are provided to further illustratethe present invention.

Example 1 Anti-Corrosion Composition

An anti-corrosion composition (approximately 1 Liter) was preparedhaving the following components:

-   -   1. 2-mercaptobenzimidazole (0.1 grams, 98% solution, available        from Sigma-Aldrich, Saint Louis, Mo.)    -   2. 3,3-dichlorobenzyl benzimidazole (0.1 gram)    -   3. n-octyl thiol (0.5 grams)    -   4. Tetrahydrofurfuryl alcohol (390 grams, available from        Sigma-Aldrich, Saint Louis, Mo.)    -   5. Zinc bromide (0.1 g)    -   6. Pluronic P-65 (2.0 grams, available from BASF)    -   7. Potassium hydroxide (2.24 grams of a 50% solution)    -   8. Water to 1 L (approximately 600 mL).

The composition was prepared by mixing the components together andletting the composition rest for about two hours.

Example 2 Thermal Stability of 2-Mercaptobenzimidazole

The active compound, 2-mercaptobenzimidazole was subjected to thermalstability analysis by Differential Scanning Calorimetry (DSC) andThermogravimetric Analysis (TGA).

DSC showed that 2-mercaptobenzimidazole has a melting point atapproximately 312° C. DSC testing showed that 2-mercaptobenzimidazolemay potentially withstand lead-free reflow, which reaches temperaturesas high as 270° C., typically 262° C.

The second sample was subjected to thermogravimetric analysis. Table 2shows the decomposition progress of the 2-mercaptobenzimidazole asdetermined by TGA.

TABLE 2 Thermogravimetric Analysis Percent Decomposition Temperature 1%221.74° C. 2% 233.75° C. 5% 254.41° C. 10% 274.14° C. 25% 302.42° C. 50%324.83° C. 75% 342.19° C.

As is apparent from Table 2, 2-mercaptobenzimidazole is stable attemperatures commonly achieved for Pb-free reflow processes, withsignificant decomposition occurring only at temperatures exceeding 300°C.

Example 4 Corrosion Testing of Panels

Ten copper panels were coated with a layer of immersion-plated silverusing AlphaSTAR®, available from Enthone Inc. The thickness of theimmersion-plated silver coating was approximately six microinch. Thedimensions of the testing coupons were 2 inch×3 inch.

The copper panels were tested for corrosion resistance. Two copperpanels, which served as the controls, were not treated with ananti-corrosion composition while eight panels were treated ananti-corrosion composition in a manner sufficient to deposit aprotective organic film.

Four panels were treated using the following anti-corrosion composition,designated Treatment #1: 2-(3,4-dichlorobenzyl)-benzimidazole (3.0 g/L),0.1 g of zinc bromide, 0.2 g of n-octyl thiol, 0.1 g of Pluronic P-65,2-ethoxyethanol (500 mL/L) and KOH (1.12 g/L).

Four panels were treated using the following anti-corrosion composition,designated Treatment #2: 2-mercaptobenzimidazole (0.1 grams, 98%solution, available from Sigma-Aldrich, Saint Louis, Mo.); 0.1 gram of3,3-dichlorobenzyl benzimidazole, 0.5 grams of n-octyl thiol,tetrahydrofurfuryl alcohol (390 grams, available from Sigma-Aldrich,Saint Louis, Mo.); 0.1 g of zinc bromide, Pluronic P-65 (2.0 grams,available from BASF); potassium hydroxide (2.24 grams of a 50%solution); and water to 1 L (approximately 600 mL). The panels weretreated by dipping with agitation the panels into the Treatmentcompositions according to the Treatment Matrix shown in Table 3. Aftertreatment, the panels were rinsed with deionized water for 30 seconds.

One control panel was subjected to 2× reflow:

TABLE 3 Treatment Matrix Controls Control, Control, 2x reflow 0x reflowSample Identification 1 3 5 7 Number Treatment #1 30 30 120 120 secondsseconds seconds seconds at 25° C. at 45° C. at 25° C. at 45° C. SampleIdentification 2 4 6 8 Number Treatment #2 30 30 120 120 seconds secondsseconds seconds at 25° C. at 45° C. at 25° C. at 45° C.

The panels were then subjected to corrosion testing by exposing thepanels first to an atmosphere containing sulfur dioxide and then to anatmosphere containing hydrogen sulfide. First, the panels were placed ina sealed glass dessicator (approximately 150 mm diameter) comprising asulfurous acid solution (6% concentration, 150 mL) for 120 minutes.Next, the panels were placed in a sealed glass dessicator (approximately150 mm diameter) comprising hydrogen sulfide (1 mL of 23.5% solution of(NH₄)₂S in 100 mL water) for 15 minutes.

The panels were visually inspected for corrosion. FIG. 2 is a photographdisplaying the panels according to the Treatment Matrix in Table 3. Thefour panels in the bottom row (pertaining to Treatment #2) exhibited theleast corrosion.

Example 5 Corrosion Testing of Coupons Through Multiple Reflows

Six copper coupons, which were copper cladded FR⁴ laminate) were coatedwith a layer of immersion silver using AlphaSTAR®, available fromEnthone Inc.

The copper coupons were tested for corrosion resistance through multiplereflows. Three copper coupons, which served as controls, were nottreated with an anti-corrosion composition. Three copper coupons weretreated with agitation using the anti-corrosion composition designatedTreatment #2 from Example 4. After treatment, the coupons were rinsedwith deionized water for 30 seconds.

The panels were subjected to reflow. Two panels were subjected to 2×reflow (one control and one that was coated with an organic protectivefilm); two panels were subjected to 3× reflow (one control and one thatwas coated with an organic protective film); and two panels weresubjected to 4× reflow (one control and one that was coated with anorganic protective film). The coupons were then subjected to corrosiontesting by exposing the panels first to an atmosphere containing sulfurdioxide (ASTM B799) and then to an atmosphere containing hydrogensulfide.

First, the panels were placed in a sealed glass dessicator(approximately 150 mm diameter) comprising a sulfurous acid solution (6%concentration, 150 mL) for 120 minutes. Next, the panels were placed ina sealed glass dessicator (approximately 150 mm diameter) comprisinghydrogen sulfide (1 mL of 23.5% solution of (NH₄)₂S in 100 mL water) for15 minutes.

The coupons were visually inspected for corrosion. FIG. 3 is aphotograph displaying the coupons, wherein the top row of coupons weresubjected to 2× reflow, the middle row of coupons were subjected to 3×reflow, and the bottom row of coupons were subjected to 4× reflow. Theleft column of coupons contained the protective organic film while theright column of coupons was left untreated.

It was apparent that the protective organic film substantially inhibitedcorrosion even after 4× reflow, while the control panels exhibitedsubstantial corrosion after 2× reflow.

Example 6 Performance Testing of Protective Organic Films

Copper coupons (2 inch×3 inch copper cladded laminate) were coated witha layer of immersion silver using AlphaSTAR®, available from EnthoneInc. The copper coupons having a layer of immersion silver thereon weresubjected to performance testing.

In the first performance test, four coupons were subjected to a wettingbalance test using eutectic tin-lead solder and flux, according toindustrial standard testing method IPC J-STD-003A. Two copper coupons,which served as controls, were not treated with an anti-corrosioncomposition. Two copper coupons were treated with agitation using theanti-corrosion composition designated Treatment #2 from Example 4. Aftertreatment, the coupons were rinsed with deionized water for 30 seconds.Two of the coupons (one treated coupon and one untreated coupons) weresubjected to wetting balance evaluation prior to reflow. Two of thecoupons (one treated coupon and one untreated coupons) were subjected towetting balance evaluation after subjected them to 2× reflow accordingto the lead-free reflow profile presented above. The results of thewetting balance test are depicted graphically in FIG. 4.

In the second performance test, four coupons were subjected to a contactresistance test using eutectic tin-lead solder and flux according toASTM B539. Two copper coupons, which served as controls, were nottreated with an anti-corrosion composition. Two copper coupons weretreated using the anti-corrosion composition designated Treatment #2from Example 4. After treatment, the coupons were rinsed with deionizedwater for 30 seconds. Two of the coupons (one treated coupon and oneuntreated coupons) were subjected to contact resistance evaluation priorto reflow. Two of the coupons (one treated coupon and one untreatedcoupons) were subjected to contact resistance evaluation after subjectedthem to 2× reflow. The results show that the contact resistance variesslightly among all samples, but the variation is very marginal.

When introducing elements of the present invention or the preferredembodiments(s) thereof, the articles “a”, “an”, “the” and “said” areintended to mean that there are one or more of the elements. The terms“comprising”, “including” and “having” are intended to be inclusive andmean that there may be additional elements other than the listedelements.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As various changes could be made in the above compositions and methodswithout departing from the scope of the invention, it is intended thatall matter contained in the above description and shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

1. A method for enhancing the corrosion resistance of an articlecomprising a silver coating deposited on a solderable copper substrate,the method comprising: exposing the copper substrate having the silvercoating thereon to an anti-corrosion composition comprising: amulti-functional molecule, wherein the multi-functional moleculescomprises at least one nitrogen-containing organic functional group thatinteracts with and protects copper surfaces and at least onesulfur-containing organic functional group that interacts with andprotects silver surfaces; an alcohol; a surfactant; and an alkaline pHadjuster.
 2. The method of claim 1 wherein the nitrogen-containingfunctional group comprises an amine.
 3. The method of claim 1 whereinthe nitrogen-containing functional group comprises one of a primaryamine, secondary amine, tertiary amine, or aromatic heterocycle.
 4. Themethod of claim 1 wherein the nitrogen-containing functional groupcomprises an azole.
 5. The method of claim 1 wherein thenitrogen-containing functional group comprises a benzimidazole.
 6. Themethod of claim 1 wherein the sulfur-containing functional groupcomprises one of a thiol, disulfide, thioether, thioaldehyde,thioketone, or aromatic heterocycle.
 7. The method of claim 3 whereinthe sulfur-containing functional group comprises one of a thiol,disulfide, thioether, thioaldehyde, thioketone, or aromatic heterocycle.8. The method of claim 5 wherein the sulfur-containing functional groupcomprises one of a thiol, disulfide, thioether, thioaldehyde,thioketone, or aromatic heterocycle.
 9. The method of claim 1 whereinthe sulfur-containing functional group comprises a thiol.
 10. The methodof claim 1 wherein the sulfur-containing functional group comprises adisulfide.
 11. The method of claim 1 wherein the multi-functionalmolecule comprises azole and thiol functional groups.
 12. The method ofclaim 1 wherein the multi-functional molecule has one of the followingstructures:

wherein R₁ is hydrocarbyl and R₂, R₃, and R₄ are nitrogen, sulfur, orcarbon; the carbon chain of the hydrocarbyl comprises between two andabout 24 carbon atoms; and any of the carbon chain of the hydrocarbyl,R₂, R₃, and R₄ may be substituted or unsubstituted.
 13. The method ofclaim 3 wherein the multi-functional molecule has one of the followingstructures:

wherein R₁ is hydrocarbyl and R₂, R₃, and R₄ are nitrogen, sulfur, orcarbon; the carbon chain of the hydrocarbyl comprises between two andabout 24 carbon atoms; and any of the carbon chain of the hydrocarbyl,R₂, R₃, and R₄ may be substituted or unsubstituted.
 14. The method ofclaim 5 wherein the multi-functional molecule has one of the followingstructures:

wherein R₁ is hydrocarbyl and R₂, R₃, and R₄ are nitrogen, sulfur, orcarbon; the carbon chain of the hydrocarbyl comprises between two andabout 24 carbon atoms; and any of the carbon chain of the hydrocarbyl,R₂, R₃, and R₄ may be substituted or unsubstituted.
 15. The method ofclaim 1 wherein the alcohol is present in a concentration between about10 mL/L and about 500 mL/L.
 16. The method of claim 1 wherein thesurfactant is non-ionic.
 17. The method of claim 1 wherein themulti-functional molecule is present in a concentration between about0.1 g/L and about 10 g/L.
 18. The method of claim 1 wherein: the alcoholis present in a concentration between about 10 mL/L and about 500 mL/L;the surfactant is non-ionic and present in a concentration between about0.01 g/L and about 2 g/L; and the multi-functional molecule is presentin a concentration between about 0.1 g/L and about 10 g/L.
 19. Themethod of claim 1 wherein: the alcohol is present in a concentrationbetween about 10 mL/L and about 500 mL/L; the surfactant is non-ionicand present in a concentration between about 0.01 g/L and about 2 g/L;and the multi-functional molecule is present in a concentration betweenabout 0.1 g/L and about 10 g/L.
 20. The method of claim 1 wherein thecomposition is free of alkali metal hydroxide.
 21. A composition forenhancing the corrosion resistance of an article comprising a silvercoating deposited on a solderable copper substrate, the compositioncomprising: a multi-functional molecule, wherein the multi-functionalmolecules comprises at least one nitrogen-containing organic functionalgroup that interacts with and protects copper surfaces and at least onesulfur-containing organic functional group that interacts with andprotects silver surfaces; an alcohol; a surfactant; and an alkaline pHadjuster.